1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly to an array substrate for the LCD device having no wavy noise problem and shorting defects and a method of fabricating the array substrate.
2. Discussion of the Related Art
Since the LCD device has characteristics of light weight, thinness and low power consumption, the LCD device has been widely used as a substitute for a display device of cathode-ray tube type.
The LCD device uses optical anisotropy and polarization properties of liquid crystal molecules to display images. The liquid crystal molecules have orientation characteristics of arrangement resulting from their thin and long shape. Thus, an arrangement direction of the liquid crystal molecules can be controlled by applying an electrical field to them. Particularly, the LCD device including a thin film transistor (TFT) as a switching element, referred to as an active matrix LCD (AM-LCD) device, has excellent characteristics of high resolution and displaying moving images. Since the LCD device includes the TFT as the switching element, it may be referred to a TFT-LCD device.
Generally, the LCD device includes an array substrate, where a TFT and a pixel electrode are formed, a color filter substrate, where a color filter layer and a common electrode are formed, and a liquid crystal layer. The array substrate and the color filter layer face and are spaced apart from each other. The liquid crystal layer is interposed therebetween.
FIG. 1 is an exploded perspective view of a conventional LCD device. As shown in FIG. 1, the LCD device includes first and second substrates 12 and 22, and a liquid crystal layer 30. The first and second substrates 12 and 22 face each other, and the liquid crystal layer 30 is interposed therebetween.
The first substrate 12 includes a gate line 14, a data line 16, a TFT “Tr”, and a pixel electrode 18, and so on. The gate line 14 and the data line 16 cross each other such that a region formed between the gate and data lines 14 and 16 is defined as a pixel region “P”. The TFT “Tr” is formed at a crossing portion between the gate and data lines 14 and 16, and the pixel electrode 18 is formed in the pixel region “P” and connected to the TFT “Tr”.
The second substrate 22 includes a black matrix 25, a color filter layer 26, and a common electrode 28. The black matrix 25 has a lattice shape to cover a non-display region of the first substrate 12, such as the gate line 14, the data line 16, the TFT “Tr”, and so on. The color filter layer 26 includes first, second, and third sub-color filters 26a, 26b, and 26c. Each of the sub-color filters 26a, 26b, and 26c has one of red, green, and blue colors “R”, “G”, and “B” and corresponds to the each pixel region “P”. The common electrode 28 is formed on the black matrix 25 and the color filter layers 26 and over an entire surface of the second substrate 22. As mentioned above, the arrangement of the liquid crystal molecules is controlled by an electric field between the pixel electrode 18 and the common electrode 28 such that an amount of transmitted light is changed. As a result, the LCD device displays images.
Though not shown in FIG. 1, to prevent the liquid crystal layer 30 being leaked, a seal pattern may be formed along edges of the first and second substrates 12 and 22. First and second alignment layers may be formed between the first substrate 12 and the liquid crystal layer 30 and between the second substrate 22 and the liquid crystal layer 30. Polarizer may be formed on at least an outer surface of the first and second substrates 12 and 22.
Moreover, the LCD device includes a backlight assembly on an outer surface of the first substrate 12 to supply light to the liquid crystal layer 30. When a scanning signal is applied to the gate line 14 to control the TFT “Tr”, a data signal is applied to the pixel electrode 18 through the data line 16 such that the electric field is induced between the pixel and common electrodes 18 and 28. As a result, the LCD device produces images using the light from the backlight assembly.
Many mask processes, which may be referred to as a photolithography process, are performed in the fabricating the array substrate for the LCD device to form a gate line, a semiconductor layer, a data line and so on. For example, the mask process includes a step of forming a material layer, a step of forming a photoresist (PR) layer on the material layer, a step of exposing the PR layer using a mask, a step of developing the PR layer to form a PR pattern, a step of etching the material layer using the PR pattern as an etching mask to form a line, an electrode, a semiconductor layer, and so on. A PR material used for the PR layer is divided into a positive type and a negative type. In the positive type, an exposed portion is removed by the step of developing. In the negative type, an exposed portion remains by the step of developing. Generally, the positive type PR material is used for a fabricating process of the array substrate. The array substrate is fabricated through a four mask process or a five mask process. For example, a five mask process for an array substrate may include a first mask process of forming a gate electrode and a gate line; a second mask process of forming a semiconductor layer over the gate electrode; a third mask process of forming a data line, a source electrode and a drain electrode; a fourth mask process of forming a passivation layer having a contact hole exposing the drain electrode; and a fifth mask process of forming a pixel electrode connected to the drain electrode through the contact hole.
Since the array substrate is fabricated through a complicated mask process, a production yield decreases. Moreover, since fabrication time and cost increase, a competitiveness of product is weakened.
Accordingly, the array substrate is fabricated through 4 mask process to increase production yield. FIG. 2 is a cross-sectional view showing an array substrate for an LCD device fabricated through a 4 mask process. As shown in FIG. 2, a first metal layer (not shown) is formed on a substrate 101 and is patterned using a first mask (not shown) to form a gate electrode 105 and a gate line (not shown). The gate electrode 105 is connected to the gate line (not shown). Next, a gate insulating layer 110, an intrinsic amorphous silicon layer (not shown), an impurity-doped amorphous silicon layer (not shown) and a second metal layer (not shown) are sequentially formed on the gate electrode (105) and the gate line (not shown). Then, the second metal layer (not shown), the impurity-doped amorphous silicon layer (not shown) and the intrinsic amorphous silicon layer (not shown) are patterned using a second mask (not shown) including one of a diffractive exposing mask and a half-tone mask to form a source electrode 130, a drain electrode 135, a data line 127 and a semiconductor layer 120 including an active layer 120a and an ohmic contact layer 120b. The half-tone mask includes a transmitting area, a blocking area and a half-transmitting area. The half-transmitting area has transmittance less than that of the transmitting area and greater than that of the blocking area. The source electrode 130 is connected to the data line 127 and spaced apart from the drain electrode 135. Next, a passivation layer 140 having a drain contact hole 145 is formed on the source electrode 130, the drain electrode 135 and the data line 127 using a third mask (not shown). The drain contact hole 145 exposes the drain electrode 135. Next, a pixel electrode connected to the drain electrode 135 through the drain contact hole 145 on the passivation layer 140 using a fourth mask (not shown).
However, there are some problems. Since the semiconductor layer 120, the source electrode 130, the drain electrode 135 and the data line 127 are formed at the same time, there are undesired patterns. Namely, a portion of the active layer 120a of the semiconductor layer 120 is not covered by the gate electrode 105 and is exposed to light from a backlight unit (not shown) under the substrate 101. Since the semiconductor layer 91 is formed of amorphous silicon, a photo leakage current is generated in the semiconductor layer 91 due to the light from the backlight unit. As a result, a property of the TFT T is degraded due to the photo leakage current.
Furthermore, an intrinsic amorphous silicon pattern 121a and an impurity-doped amorphous silicon pattern 121b are formed under the data line 127. The intrinsic amorphous silicon pattern 121a protrudes beyond the data line 127. The protruding portion of the intrinsic amorphous silicon pattern 121a is exposed to the light from the backlight unit or an ambient light. Since the intrinsic amorphous silicon pattern 121a is also formed of amorphous silicon, a light leakage current is generated in the intrinsic amorphous silicon pattern 121a. The light leakage current causes a coupling of signals in the data line 127 and the pixel electrode 150 to generate deterioration such as a wavy noise when displaying images. A black matrix (not shown) designed to cover the protruding portion of the intrinsic amorphous silicon pattern 121a reduces aperture ratio of the LCD device.
Moreover, there are shorting defects between the data line and the pixel electrode. In more detail, since the pixel electrode 150 is closest to the data line 127 to maximize aperture ratio, the shorting defects is generated between the data line 127 and the pixel electrode 150. It is caused by pattering error of the data line 127 and the pixel electrode 150 and the passivation layer 140 between the data line 127 and the pixel electrode 150. A repairing process is performed to overcome the shorting defects. For example, a contacting portion of the data line 127 and the pixel electrode 150 is cut using a laser. The repairing process increases fabricating time and decreases production yield.